The invention relates to a process for making polyethylene. The process, which uses catalysts having a bridged indenoindolyl ligand with xe2x80x9copen architecture,xe2x80x9d is valuable for making polyethylene with good flow properties.
While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry""s future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of xcex1-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Single-site olefin polymerization catalysts having xe2x80x9copen architecturexe2x80x9d are generally known. Examples include the so-called xe2x80x9cconstrained geometryxe2x80x9d catalysts developed by scientists at Dow Chemical Company (see, e.g., U.S. Pat. No. 5,064,802), which have been used to produce a variety of polyolefins. xe2x80x9cOpen architecturexe2x80x9d catalysts differ structurally from ordinary bridged metallocenes, which have a bridged pair of pi-electron donors. In open architecture catalysts, only one group of the bridged ligand donates pi electrons to the metal; the other group is sigma bonded to the metal. An advantage of this type of bridging is thought to be a more open or exposed locus for olefin complexation and chain propagation when the complex becomes catalytically active. Simple examples of complexes with open architecture are tert-butylamido(cyclopentadienyl)dimethylsilylzirconium dichloride and methylamido(cyclopentadienyl)-1,2-ethanediyltitanium dimethyl: 
Organometallic complexes that incorporate xe2x80x9cindenoindolylxe2x80x9d ligands are known (see U.S. Pat. Nos. 6,232,260 and 6,451,724). The ""260 patent demonstrates the use of non-bridged bis(indenoindolyl) complexes for making HDPE in a slurry polymerization. Versatility is an advantage of the complexes; by modifying the starting materials, a wide variety of indenoindolyl complexes can be prepared. xe2x80x9cOpen architecturexe2x80x9d complexes are neither prepared nor specifically discussed. The ""724 patent (xe2x80x9cNifant""evxe2x80x9d) teaches the use of bridged indenoindolyl complexes as catalysts for making polyolefins, including polypropylene, HDPE and LLDPE. The complexes disclosed by Nifant""ev do not have open architecture.
PCT Int. Appl. WO 01/53360 (Resconi et al.) discloses bridged indenoindolyl complexes having open architecture and their use to produce substantially amorphous propylene-based polymers. All of the complexes are used only to make propylene polymers; their use to produce ethylene polymers is not disclosed.
U.S. Pat. No. 6,559,251 discloses a process to make low density ethylene copolymers with catalysts having a bridged indenoindolyl ligand with open architecture. The examples use a t-butyl substituted amido ligand and the molecular weight is extremely high and ranges from 784,000 in Example 1 which includes hydrogen in the polymerization to 1,180,000 in Example 4 which uses a supported catalyst and no hydrogen. These molecular weights are so high that the copolymer would have poor flow properties and would have limited usefulness for many applications. There is no teaching as to how to control the molecular weight or of the criticality of the amido substituents.
Copending application Ser. No. 10/382,233 discloses that indeno[1,2-b]indolyl catalysts provide exceptional activities in the preparation of elastomeric polypropylene and ethylene copolymers. An open architecture complex incorporating a C8 hydrocarbyl amido group is used in their Example 15 in a propylene polymerization. However, the molecular weight of the polypropylene is higher (944,000) than in their Example 12 where a complex containing a C4 hydrocarbyl amido group is used to obtain polypropylene with molecular weight of 736,000. Thus, the results suggest, if anything, that using a longer alkyl chain as the amido substituent will hinder rather than enhance polyolefin flow properties.
As noted earlier, the indenoindolyl framework is versatile. The need continues, however, for new ways to make polyolefinsxe2x80x94especially ethylene copolymersxe2x80x94with good flow properties. In particular, it is difficult to make ethylene copolymers having melt indices greater than about 0.5 using known processes. On the other hand, ethylene copolymers having good flow properties are valuable for several applications requiring certain melt processing techniques such as injection molding. The industry would also benefit from the availability of new processes that capitalize on the inherent flexibility of the indenoindolyl framework.
The invention is a process for making polyethylene. The process comprises polymerizing ethylene in the presence of a catalyst system comprising an activator and a supported organometallic complex. The complex, which has xe2x80x9copen architecture,xe2x80x9d includes a Group 4 to 6 transition metal and a bridged indenoindolyl ligand. We surprisingly found that modifying the open architecture complex to incorporate a C6-C20 hydrocarbyl amido group enables the production of polyethylene having improved flow and good rheological properties while maintaining high catalyst activity.